How do I find a toaster that doesn't "leak" crumbs from the bottom?

You could put it into a tray of some sort but unless you 3D-print one, its hard to find a tray that exactly fits it.

Finding a tray that fits is possible with a bit of smart shopping or DIY.

One way to find an tray that exactly fits under your toaster is to trace around your toaster on a piece of paper and cut out the tracing to bring with you to the store. Go to a flea market or large secondhand store (eg, in the US, find a larger branch of Salvation Army or Goodwill). The advantage of shopping secondhand is they'll have a hodgepodge of different sizes and shapes, including sizes and shapes that are no longer widely sold new. Plus it'll be cheap. Alternatively, you can try shopping at yard sales, but you'll probably have to visit more than one to find the right pan.

If you're up for a little DIY, get one of those cheap aluminum foil pans that are meant to be single-use, like the kind you would use to make lasagna to take to a potluck. Get the size closest to your toaster's size. Use pliers and tin snips or an old pair of scissors to reshape the pan to exactly the right size. If you cut the metal, make sure you leave enough extra so you can fold over the cut edge so you won't cut yourself on it.

Many toasters have removable crumb trays, where crumbs collect instead of falling out the bottom. You then empty the tray once a week or so.

Not all such trays are equally effective, so I suggest using your usual sources of reviews to find the ones which are (Amazon, Consumer Reports, etc.)